A continuing survey of Medline entries is being conducted to estimate the incidence of recognized internal periodicity in molecular sequence data. Text word searching (using IRX) for the conjunction of the terms "protein" and "repetitive" and "sequence" yields approx. 50 new citations month. Statistics with respect to repeat unit length and copy number are being collected in a database of repetitive sequence proteins. These data are being used to discover general principles of repeat structure, function andevolution and also as a "training set" for the design of new self-comparison algorithms for detecting and characterizing internal periodicity in sequence data. Studies of this data have led to the following observations. Repetitive protein sequences are widespread in nature and serve many biological purposes. As many as 10 to 20% of known protein sequences have demonstrable internal repeat(s). A sampling of biological functions includes: timekeeping in the central nervous system, ice nucleation and antifreeze activities in bacteria and arctic fish, nutrient storage in plant seeds, antigenic variation in malarial parasites, cellular and humoral immunity, complement fixation and the clotting cascade, serum lipid transport and cholesterol metabolism, hormone action and signal transduction, cell-cell recognition in embryogenesis and regulation of gene expression. Repeat unit do not appear to be quantized; i.e., there is a continuous variation of unit lengths in natural sequences, from simple homopolymers and alternating copolymers to duplications of entire globular domains. Furthermore as expected for sequences of finite length, there appears to be an inverse relationship between repeat unit length and copy number. In addition to approaching repeats as a general phenomenon, I continue to study in detail specific types of repeats. These include "Src homology" domains in oncogene products and signal transduction pathways, "WD" repeats in the beta-transducin family, and two new repetitive sequence motifs in the alpha and beta subunits of farnesyl- and geranylgeranyl transferases.